The generation of contrast in an MRI image through irradiation of mobile protons in the tissues has already been reported in U.S. Pat. No. 5,050,609. The use of exogenous contrast agents (originally called CEDST agents, and now called CEST agents) containing at least one mobile proton in exchange with water was disclosed in WO 00/66180. That document does not expressly refer to the possibility of using paramagnetic complexes as CEST agents. The preferred compounds are diamagnetic molecules such as sugars, aminoacids, heterocyclic compounds, nucleosides, imidazoles and derivatives, guanidine, etc.
The same document also claims methods for in vitro and in vivo determination of the pH, temperature and metabolite concentration by administering a contrast agent having two sets of magnetically non-equivalent mobile protons. Selective irradiation of the two pools and application of a ratiometric method allows a transfer effect to be measured independently of the concentration of contrast medium. Saturation transfer implies that the mobile protons of the CEST agent do not coalesce with the bulk water, which means that the separation between the resonance frequencies of the two sets of mobile protons must be greater than the exchange rate. Moreover, the extent of the transfer is directly proportional to the exchange rate of the protons of the agent, which means that an increase in separation of the resonance frequencies of the two exchanging signals increases the efficiency of saturation transfer. The use of CEST paramagnetic contrast agents was therefore considered in WO 02/43775, which claims the use of paramagnetic complexes having mobile protons whose saturation transfer (which is better than that of the diamagnetic compounds exemplified in U.S. Pat. No. 5,050,609) can be sensitive to parameters of diagnostic interest such as pH, temperature, metabolite concentration, etc.
However, the possibility of using as CEST agents systems constituted by non-covalent adducts formed by a substrate having protons in exchange with bulk water (including types similar to those exemplified in U.S. Pat. No. 5,050,609) and by a paramagnetic complex able to increase the separation of their resonance frequencies off the bulk water is not considered in the above-mentioned documents or in the literature. With this approach, the paramagnetic agent will be chosen on the basis of its Shift Reagent (SR) characteristics in relation to the properties of the compound containing the exchangeable protons.